This invention relates to the dispersion of particulate materials in aqueous systems. More particularly, the invention concerns the dispersion of particulate solids in aqueous media to prevent deposition of the solids onto surfaces contacted by the media.
A continuing problem of industrial systems is the deposition of particulates from aqueous streams onto solid surfaces contacted by the streams, such as pipes, heat exchangers (for both heating and cooling), reactors, storage tanks, boilers, and the like. This condition is sometimes described as "fouling" and/or "scaling." The particulates may have a natural origin, such as silt, clays and other materials, both organic and inorganic, carried by rivers and streams. They may also constitute materials formed within an industrial system, which exceed the solubility limits of the system, such as scale, or materials added to a system, such as pigments, clay, starch, and other modifiers. Over time such materials will deposit on solid surfaces and cause constriction and/or reduced heat transfer, thereby leading to inefficient operation and eventual malfunctioning of a heat transfer or other system serviced by the aqueous medium.
The dispersion of inorganics such as kaolin-type clay minerals presents a special problem because studies have shown that the good adsorption of a dispersant material on the clay (which adsorption is a basic condition for dispersion of the clay particles) depends on the part of the clay crystal to which the dispersant molecule is attracted, whether or not the adsorption is single or multi-layered, and other more apparent considerations, such as polarity of the dispersion medium, temperature, and mobility of the system being treated. For example, kaolin clay crystal edges generally are more polar than the crystal surfaces; consequently, adsorption may occur more readily on the edges, depending on the polarity of the dispersant. Such adsorption is less favorable for dispersion than adsorption on crystal surfaces. Single layer adsorption is preferred over multi-layered adsorption, to avoid bridging between the crystals and weakening of the charge repulsion between the crystals (the maximization of which is the objective in dispersion). Accordingly, selection of a good dispersant for clay minerals is a highly empirical art.
Low molecular weight polymers such as styrenemaleic anhydride polymers have been developed as dispersants in aqueous heat exchange systems, as described, for example, in U.S. Pat. Nos. 2,723,956, 3,948,792, 4,288,327 and 4,374,733.
In scale formation calcium salts are precipitated, often accompanied by oxidation and other forms of corrosion of metal surfaces. U.S. Pat. No. 4,328,180 to G. D. Hansen and its division, U.S. 4,384,979, describe formulations useful for such purposes, including the use of a low molecular weight polymer of acrylic acid, terminated by thioglycolic acid. The latter material is more particularly described, with synthesis and use as a scale inhibitor, in U.S. Pat. No. 3,665,035 and its division 3,756,257, both to Rice et al.
Despite the efficacy of the compositions disclosed in the foregoing patents for control of corrosion of metal surfaces in contact with cooling or for control of deposition of materials from various aqueous systems, there remains a need in these and other areas of water treatment for enhancement of the dispersion of particulate materials, particularly of kaolin-type clay minerals, and thereby to prevent deposition onto surfaces of industrial systems. "Dispersion" in the context of this invention means maximizing the repulsion between solid particles in an aqueous system, resulting in reduced deposition of particulate matter on surfaces in contact with aqueous systems. The improved dispersion is sometimes evidenced by improved suspension of the particulates but can also result in solubilization, emulsification or other liquefaction.